yii2/framework/requirements/requirements.php

<?php
/**
 * These are the Yii core requirements for the [[YiiRequirementChecker]] instance.
 * These requirements are mandatory for any Yii application.
 *
 * @var YiiRequirementChecker $this
 */
return array(
    array(
        'name' => 'PHP version',
        'mandatory' => true,
        'condition' => version_compare(PHP_VERSION, '5.4.0', '>='),
        'by' => '<a href="http://www.yiiframework.com">Yii Framework</a>',
        'memo' => 'PHP 5.4.0 or higher is required.',
    ),
    array(
        'name' => 'Reflection extension',
        'mandatory' => true,
        'condition' => class_exists('Reflection', false),
        'by' => '<a href="http://www.yiiframework.com">Yii Framework</a>',
    ),
    array(
        'name' => 'PCRE extension',
        'mandatory' => true,
        'condition' => extension_loaded('pcre'),
        'by' => '<a href="http://www.yiiframework.com">Yii Framework</a>',
    ),
    array(
        'name' => 'SPL extension',
        'mandatory' => true,
        'condition' => extension_loaded('SPL'),
        'by' => '<a href="http://www.yiiframework.com">Yii Framework</a>',
    ),
    array(
        'name' => 'MBString extension',
        'mandatory' => true,
        'condition' => extension_loaded('mbstring'),
        'by' => '<a href="http://www.php.net/manual/en/book.mbstring.php">Multibyte string</a> processing',
        'memo' => 'Required for multibyte encoding string processing.'
    ),
    array(
        'name' => 'Intl extension',
        'mandatory' => false,
        'condition' => $this->checkPhpExtensionVersion('intl', '1.0.2', '>='),
        'by' => '<a href="http://www.php.net/manual/en/book.intl.php">Internationalization</a> support',
        'memo' => 'PHP Intl extension 1.0.2 or higher is required when you want to use advanced parameters formatting
        in <code>Yii::t()</code>, non-latin languages with <code>Inflector::slug()</code>,
        <abbr title="Internationalized domain names">IDN</abbr>-feature of
        <code>EmailValidator</code> or <code>UrlValidator</code> or the <code>yii\i18n\Formatter</code> class.'
    ),
    array(
        'name' => 'Fileinfo extension',
        'mandatory' => true,
        'condition' => extension_loaded('fileinfo'),
        'by' => '<a href="http://www.php.net/manual/en/book.fileinfo.php">File Information</a>',
        'memo' => 'Required for files upload to detect correct file mime-types.'
    ),
);